Method for electrowinning molybdenum from molten electrolytes



Jan. 1, 1963 J. B. ZADRA ETAL METHOD FOR ELECTROWINNING MOLYBDENUM FROM MOLTEN ELECTROLYTES Filed Feb. 6, 1961 INVENTORS JOHN B. ZADRA HROLD J HENEV JOHN M. GOMES BY (EM M W WORNEYS 34371523 METHOD FOR ELECTROWINNIN G MOLYBDE- NUM FRGM MOLTEN ELECTROLYTES John B. Zadra, Harold J. Heinen, and John M. Gomes,

Reno, Nev., assignors to the United States of America as represented by the Secretary of the Interior Filed Feb. 6, 1961, Ser. No. 87,502 6 Claims. (Ci. 204-64) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for government-al purposes without the payment of royalties thereon or therefor.

The present invention relates to the production of pure molybdenum in massive form by the electrolysis of a fused electrolyte.

Currently, the most important commercial procedure for producing molybdenum metal is by the hydrogen reduction of high purity molybdic oxide or ammonium molybdate in an electric furnace. Molybdenum powder produced thereby contains approximately 0.25 to 0.30 percent oxygen. It is then converted to massive metal and reduced in oxygen content by sintering in a hydrogen atmosphere, or by are melting. These methods are costly, and in addition, -are accompanied by excessive fume losses and other technical difficultes.

As shown in Bureau of Mnes Information Circular 7723 by Campbell and Jones, issued July 1955, entitled A Survey of the Literature on the Electrodeposition of Molybdenum, it has been proposed to electrodeposit molybdenum from many aqueous or fused salt electrolytic systems. Substantially all of the methods disclosed sutfer from the serious disadvantage that the metal deposits are extremely thin and are usually contaminated with the lower oxides and hydroxides of the molybdenum.

The Senderoif and Brenner Patent 2,715,093 describes the formation of massive molybdenum 'by electrolysis of a fused bath of a salt such as K MoCl However, this process requires rigid reaction conditions and control, such as special high purity alkali metal hexahalomolybdate, inert atmosphere and the exclusion of all traces of oxygen and moisture, resulting necessarly in a relatively high cost process.

Patent 2,960,4S 1, issued November 15, 1960 to Slatin, recites the production of molybdenum by the electrolysis of a CaC1 -CaO-Mo electrolyte. The presence of -alkali metal compounds is stated by the patentee to be harmful. Also, MoO is stated as not being as compatible with his CaCI -CaO bath as WO (col. 6, line 61).

It is the main object of this invention to provide -a simple, practical method for electrowinning high purity molybdenum from its compounds.

A further object of this invention is to produce high purity molybdenum from its impure compounds by fused salt electrolysis in an open cell under atmospheric pressures.

A further object of this invention is to produce high purity molybdenum metal substantially free from oxygen and phosphorus.

A 'still further object of the invention is to produce massive deposits of pure crystalline molybdenum metal.

A further object of this invention is to provide a method for the electrolytic recovery of molybdenum from a bath containing sodium.

A further object of this invention is to provide for efficient spacng of the electrodes to produce highpurity molybdenum in a fused salt electrolytic process.

Further objects and advantages will become appa'ent from the rest of the specification.

In brief, our method consists in making a molybdenum electrolyte by fusing a molybdenum compound &071523 iee such as molybdenum oxide, with a mixture of alkali or alkaline earth metal pyrophosphates, borates and halides in 'a crucible. Passage of a direct current of proper our-- rent density therethrough results in the deposition of high purity crystalline molybdenum on the cathode.

While the exact theoretical explanation for the successful production of molybdenum is not known at present, it `Would appear to be due to the particular electrolyte employed. The essential Components of the electrolyte are a molybdenum compound, a pyrophosphate 'and .a halide. Molybdic oxide MoO is usually the most convenient molybdenum compound to use. However, other moly bdenum compounds may be employed, such as the molybdenum chloride for example. The purity of the compound is not significant in this process, and in fact one of the advantages claimed is that impure molybdenum compounds may be employed.

Sodium pyrophospha te is the most readily obta'nable pyrophosphate, and is therefore the one ordinarily used. Similarly, sodiurn chloride is the chloride usually empioyed. However, the sodiurn in these compounds may be replaced by lithium, potassium, rubidium, cesiurn, calcium, -barium, or strontium, and the halogen in the halide may be uorine, bromine, or iodine, as well as chlorine. The addition of sodium tetrabora te to the electrolyte makes'the ratio of sodium chloride to sodium pyrophosphate less critical for the production of molybdenum essentially free of phosphorus. The sodurn tetraborate also facilitates the uxing of silicious impurities that may be present in the molybdenum compounds ernployed. Sodium metaborate may be substituted for the tetraborate, and the sodium may be replaced by other -alkal metals.

In percent, the molybdenum content of the electrolyte may range from 2.0 to 15.0; pyrophosphate from 15 to tetraborate from 0 to 60; and halide from 15 to 85. The preferred ranges are: Molybdenum 4 to 8; pyrophosphate 30 to 50; tetraborate O to 20; and halide 30 to SO. All the foregoing and subsequent percentages refer to percent by weight.

The following table lists four compositions, having Components within the -above ranges, found satisfactory in the process:

TABLE 1 MoO Na4P2O7 NagB o NaBOz Eutectic of NaCl-NaF NFP menus The temperature of the electrowinning reaction may range from 800 to l200 C. A preferred range is from 950 C. to 1050 C., at which the bath is very fluid. Lower temperatures result in electrodeposits consisting largely of molybdenum dioxide and a phosphorus containing compound, presumably a phosphide, with little, if any, crystals of metallic molybdenurn.

Current density on the cathode may range from 20 to 280 amperes per square decirneter (a./dm. based on the original dimensions of the electrode. Since the cathode 'surface area changes during the electrolysis, the actual current density during operation is diicult to determine. However, that based on the original dimensions is adequate for the present purposes. The current density on the `anode is not critical, and is determined by its surface area once the current density of the cathodc is selected.

Current efliciencies of around percent are obtained over the whole range. In general, this factor appears to have less influence on the results achieved than either bath temperature or composition of the electrolyte.

FEG. l illustrates a cross-sectional View of the electrolytic cell showing crucible l, furnace 2, graphite block 3 and cathode 4.

The electrolytic cell or crucible may be made of graphite silicon nitride or silieon carbide. However, the preferred material of Construction is graphite. Usually the graphite cell is made the anode.

The cathode may be made of any suitable material, such as iron, molybdenum or carbon. Graphite is very Satisfactory for the purpose. Any material that is not attacked may be employed as the anode in electro-Winning. In the following example the graphite electrolysis cell was employed as the anode.

Although an all-graphite cell is used, carbon contamination of the electrodeposited molybdenum may be practically eliminated by a proper spacing of the electrodes, use of optimurn coneentration of molybdenum for the particular electrolyte employed, and by the hydraulic classication or washing of the processed crystalline metal.

For example, an electrolyte of the second composition in Table l, contains molybdenum within the optinum range for excellent over-all electrolytic performance. A l-inch diameter graphite cathode was centrally positioned in a 3-inch diameter graphite cell and lowered in the bath to within two inches from the bottom of the cell. The resulting electrowon molybdenum metal averaged 0.01 percent carbon. in other runs carbon contamination was lowered to parts per million.

The following example illustrates one specific embodiment of the method, and the invention is not to be construed as being limited thereto.

A graphite crucible 7 inches deep and 3 inches inside diameter preheated to a temperature of about 750 C., about the melting point of the mixture and then charged with 900 grams of a salt-molybdic oXide electrolyte mixture consisting of 41.6% sodium pyrophosphate, 333% sodium chloride, 16.7% sodium tetraborate, and &4% molybdic trioxide. The crucible, open to the atmosphere, was placed in a furnace under atmospheric pressure and connected to the positive terminal or' a DC. power source to serve as the anode. After heating for about 30 minutes at a selected temperature of between 950 to l050 C. to stabilize the conditions, a graphite cathode 1 inch in diameter, connected to the negative terminal of the D.C. power source, was centrally positioned and lowered into the molten bath to within 1% to 2 inches from the bottom of the cell and electrolysis was started. Proper spacing of the cathode in the cell is important in order to eliminate codeposition of crystalline MoO and to prevent short circuiting. It has been found that the spacng between the immersed tip of the cathode and the bottom of the cell must be greater than 1% inches because of the downward crystal growth of the molybdenum up to one inch beyond the cathode tip. A shorter spacing than 1% inches usually results in some codeposition of crystalliue MoO Current density on the cathode was maintained at between 80 and 90 a./clm. and the bath temperatura was about 1000 C. Two hours of current passage Ws required to complete the electrolysis.

The cathode was then removed from the bath, allowed to cool in the air, and then immersed in water. Adhering solidified electrolyte dissolved in the water and was removed from the deposited metal. The molybdenum metal was then removed from the cathode and further treated with boiling dilute hydrochloric acid to dissolve any adher ing calcium or phosphate compounds. After water washing to remove the acid, ultra fine particles of carbon derived from the electrodes were removed by hydraulic classification, leaving high purity dendritic crystals of molybdenum metal.

The following table gives a comparative analysis of the Mo0 feed and the molybdenum metal produced there- 4. from. This shows the high purity of the product obtained.

TABLE 2 MoO feed, Electrowon Element percent molybdenum,

perserit Analysis of Feed Material and Electrolyt'c Molybdenum P'odced Therefrom and Compamz've Analysis of Hydi'ogen Redzced Molybdenum Powder Hydrogen- Reduced, C.P. Molybdenum Powder, percent Feed Material percent Eleetrolytic Molybdenum percent Elements 1 Not determined.

From the table, it can be seen that electrolytic molybdenum compares favorably with commercial hydrogenreduced molybdenum. A portion of each of the two metals are melted into ingots and tested for hardness. The.- Vickers hardness number (VHN) for electrolytic mo-- lybdenum was 186 as Compared to 170-172 for the commercial molybdenum metal. This is believed attributable to the phosphorus content in the metal. Subsequent experiments have produced phosphorus free metals by a more careful control of conditions.

Various modifications of the process may be made' within the scope of the invention, as defined by the following claims.

We claim:

1. A process for obtaining massive molybdenum which comprises: fusing a mixture of about 41.6% sodium pyrophosphate, about 333% sodium chloride, about 16.7% sodium tetraborate, and about &4% molybdic trioxide to form an electrolyte bath, heating the electrolyteto a temperature of from about 950 C. to about 1050 C., electrolyzing the said electrolyte at a current density on the cathode of from 20 a./dm. to about 280 a./dm.

2. The method of claim 1, wheren the temperature of the bath is about 1000 C. and the current density on Ll the cathode is between about 80 a./dm. to about 90 a./dm.

3. A process for obtaining massive molybdenum which comprises: fusing a mixture of a molybdenum compound, a metal pyrophosphate, a metal tetraborate, and a metal halide, said metal being a member of the class consisting of lithium, sodium, potassium, rubidium, cesium, calcium, barium, and strontium, in a vessel serving as an anode to form an electrolyte bath, heating the said electrolyte to a temperature of from about 950 C. to about 1050 C., inserting a cathode rod in the said bath, and electrolyzing the electrolyte while maintaining the lower end of the said cathode rod a minimum distance of about 1% inches from the bottom of the anode vessel, the amount of said molybdenum compound being sufiicient to provide a bath containing about 2 to 15% of molybdenum.

4. An electrolyte for producing molybdenum metal by electrolysis consisting of the product obtained by fusing together a mixture of a molybdenum compound, a metal pyrophosphate, a metal tetraborate, and a metal halide, `said metal being a member of the class consisting of lithium, sodium, potassium, rubidium, cesium, calcium, barium, and strontium, the amount of said molybdenum compound being sufi'icient to provide a bath containing about 2 to 15% of molybdenum.

5. An electrolyte for producing molybdenum metal by electrolysis consisting of the product obtained by fusing together a mixture of about 416% sodium py rophosphate, about 333% sodium chloride, about 16.7% sodium tetraborate, and about &4% molybdic trioxide.

References Cted in the file of this patent UNITED STATES PATENTS 2,554,527 Fink et al. May 29, 1951 FOREIGN PATENTS 203,614 Australia Sept. 13, 1956 OTHER REFERENCES Leo et al.: Transactons of the Electro-Chemical Society, Volume 66, pages 461, 469 (1934).

Fink et al.: The Electro Chemical Society, Preprint 84-20, October 14, 1943, pages 197-226.

Pink et al.: Chem. Trade Journal and Chemical Engineer, January 14, 1944, pages 33-34.

Bureau of Mines, Report No. 5554, Electrowinning Tungsten and Associated Molybdenum From Scheelite," Zadra et al. (1959). 

1. A PROCESS FOR OBTAINING MASSIVE MOLYBDENUM WHICH COMPRISES: FUSING A MIXTURE OF ABOUT 41.6% SODIUM PYROPHOSPHATE, ABOUT 33.3% SODIUM CHLORIDE, ABOUT 16.7% SODIUM TETRABORATE, AND ABOUT 8.4% MOLYBDIC TRIOXIDE TO FORM AN ELECTROLYTE BATH, HEATING THE ELECTROLYTE TO A TEMPERATURE OF FROM ABOUT 950*C. TO ABOUT 1050* C., ELECTROLYZING THE SAID ELECTROLYTE AT A CURRENT DENSITY ON THE CATHODE OF FROM 20 A./DM.2 TO ABOUT 280 A./DM.2. 